You don't say what type of tank or what type of pleco or what type of gourami you have. If they are both mannish, and have both decided that they want the same article of the tank, they could be squabbling about that.

Answers: Some plecos develop the tendency of trying to feed stale other fishes slime coat. They learn this behavior especially if at one time they be in a tank with dying fishes(they swot up other fish can be a nice meal)..
Your gourami is probably trying to defend himself from the pleco approaching him, plecos do grow quite big!! I had at one time literally cull two plecos and bushynose plecos as economically. For the most part they tend to behave well but plecos do inevitability lots of food...